512 LECTURE LIU. 



nearest parts of the neutral body, so that it is accumulated at the opposite 

 extremity ; while the matter, which is left deficient, attracts the redundant 

 fluid of the first hody, in such a manner as to cause it to be more con- 

 densed in the neighbourhood of the second than elsewhere ; and hence the 

 fluid of this body is driven still further off, and all the effects- are re- 

 doubled. The attraction of the redundant fluid of the electrified body for 

 the redundant matter of the neutral body, is stronger than its repulsion 

 for the fluid which has been expelled from it, in proportion as the square 

 of the mean distance of the matter is smaller than that of the mean dis- 

 tance of the fluid : so that in all such cases of induced electricity, an 

 attraction is produced between the bodies concerned. And a similar 

 attraction will happen, under contrary circumstances, when a neutral 

 body and a body negatively electrified, approach each other. 



The state of induced electricity may be illustrated by placing a long 

 conductor at a little distance from an electrified substance, and directed 

 towards it ; and by suspending pith balls or other light bodies from it, in 

 pairs, at different parts of its length : these will repel each other, from 

 being similarly electrified, at the two ends, which are in contrary states 

 of electricity, while at a certain point towards the middle, they will 

 remain at rest, the conductor being here perfectly neutral. It was 

 from the situation of this point that Lord Stanhope* first inferred the true 

 law of the electric attractions and repulsions, although Mr. Cavendish^ 

 had before suggested the same law as the most probable supposition. 



The attraction thus exerted by an electrified body upon neutral sub- 

 stances, is strong enough, if they are sufficiently light, to overcome their 

 gravitation, and to draw them up from a table at some little distance : 

 upon touching the electrified body, if it is a conductor, they receive a 

 quantity of electricity from it, and are again repelled, until they are 

 deprived of their electricity by contact with some other substance, which, 

 if sufficiently near to the first, is usually in a contrary state, and there- 

 fore renders them still more capable of returning, when they have touched 

 it, to the first substance, in consequence of an increased attraction, assisted 

 also by a new repulsion. This alternation has been applied to the con- 

 struction of several electrical toys ; a little hammer, for example, has been 

 made to play between two bells ; and this instrument has been employed 

 for giving notice of any change of the electrical state of the atmosphere. 

 The repulsion, which takes place between two bodies, in a similar state of 

 electricity, is the cause of the currents of air which always accompany the 

 discharge of electricity, whether negative or positive, from pointed sub- 

 stances ; each particle of air, as soon as it has received its electricity from 

 the point, being immediately repelled by it ; and this current has also been 

 suppossd to facilitate the escape of the electricity, by bringing a continual 

 succession of particles not already overcharged. 



If two bodies approach each other, electrified either positively or 

 negatively in different degrees, they will either repel or attract each other, 

 according to their distance : when they are very remote, they exhibit a 



* Lord Mahon's Principles of Electr. 4to, Lond. 1779. * , 



f On the Principal Phenomena of Electr. Ph. Tr. 1771, p. 584. 



